1. Technical Field
The present invention relates to a receiving apparatus, a sending apparatus and a transmission system for transmitting music information and its compressed music data digitally between digital apparatuses.
2. Background Art
International standards such as IEC 60958 and IEC 61937 are methods for serial digital transmission of conventional music information or music data.
The IEC 60958 is a method used at the time when transmitting 2ch linear PCM data and widely used for digital data transmission from CDs and DVDs. Also, the IEC 61937 is a method used at the time of communicating data that is non-linear PCM like compressed music data such as MPEG and has been used recently in the case of outputting DVD compression multi channel voice, connecting to the external decoder amplifier so as to decode and play back the voice. An object of the above-mentioned conventional music information and music data transmission method is to easily transmit and play back the transmitted compressed music data.
Recently, MPEG standardizes the ISO 13818-7 MPEG2 AAC for enabling high quality playback in the case of low bit rate at the level of 128 kbps stereo as an international standard. On the other hand, in the MPEG2 AAC, in the case of lower bit rate at the level of 48 kbps stereo, human sensitivity to the sound of high frequency around 10 kHz or more becomes lower than the sensitivity to the low frequency, reduces information distribution to the high components and, as a result, becomes playback sound with a narrow band. Coding information amount of around 48 kbps stereo bit rate reduces the band that can be quantized and coded retaining sound quality to around 10 kHz at most. Here, MPEG suggests a method for enabling playback of high band by adding little information amount to a coded stream that adds playback sound with a narrow band like this. There is a method (ISO 13818-7:2003/AMD1) that is being standardized as the Spectral Band Replication (AAC SBR), the method is for retaining information of high frequency band in an area inside bit stream data even in the case of a low bit rate, reconstructing a part of high frequency band using the high frequency information at a decoding side even in the case where a basic part is compressed using a low sampling frequency so as to play back the bit stream.
FIG. 1 is a diagram showing an example of connecting apparatuses in the case of performing data transmission.
The music data transmission system 900 comprises a music data sending apparatus 910 for transmitting music data using a predetermined interface (for example, IEC 61937), a music data receiving apparatus 920 for receiving music data and a transmission path 930 for connecting these apparatuses.
FIG. 2A is a diagram showing the format structure of the data burst 80 to be transmitted from the music data sending apparatus 910.
As shown in FIG. 2A, each data burst 80 comprises a burst preamble 81 and a burst payload 82 that follows this burst preamble 81.
FIG. 2B is a diagram showing the format structure of the burst preamble 81.
The burst preamble 81 comprises a Pa811 for indicating a synchronization word field, a Pb812, a Pc813 for indicating the type of data and a field for providing information for a receiver (burst information) and a Pd814 for indicating a field for providing the length of the burst payload.
FIG. 3 is a diagram showing a structural example of burst information.
As to bit 0-4 of the Pc813, value 7 indicates “MPEG 2AAC ADTS”, but value 0 to 6 and 8 to 31 indicate “in accordance with IEC 61937”. Also, bit 5-15 indicates “in accordance with IEC 61937”.
FIG. 4 is a diagram showing a structural example as to the bit 8-12 of the burst information.
As to bit 8-12 of the Pc813, value 0 indicates “No indication”, value 1 indicates “LC profile”, value 2 to 3 indicates “Reserved for Future profile”, and value 4 to 31 indicates “Reserved”.
In other words, conventional burst information does not distinguish MPEG 2 AAC from MPEG 2 AAC SBR.
FIG. 2C is a diagram showing the bit stream structure of the MPEG2 AAC stored in the burst payload 82.
As shown in FIG. 2C, the bit stream of the MPEG2 AAC stored in the burst payload 82 comprises a stream header 821 and a basic compression stream 822 of the compressed music data.
FIG. 2D is a diagram showing the bit stream structure of the MPEG2 AAC SBR stored in the burst payload 82.
As shown in FIG. 2D, the bit stream of the MPEG2 AAC SBR comprises a stream header 821, a basic compression stream 822 and a high frequency information parameter 823. The high frequency information parameter 823 is recorded using a part (for example, fill element) of the last part of the basic compression stream 822.
The music data receiving apparatus 920 analyzes this bit stream and reconstructs the high frequency band using the high frequency information parameter, but it can decode only the basic compression stream 822 and ignore the high frequency parameter 823 even in the case where the music data receiving apparatus 920 can perform only a normal AAC decoding processing and cannot perform an SBR processing, which enables maintaining upward compatibility with the MPEG2 AAC.
However, in the case of sending this information using the IEC 61937 standard, frequency band is restricted and this information is compressed in most cases because the part of basic compression stream 822 has a stricter limit in information amount in the case where it is compressed using a lower bit rate of 48 kbps stereo or the like than in the case where it is compressed using a higher bit rate of 128 kbps or the like. In the case where the sampling frequency of original sound is 48 kHz, it is compressed by down sampling to the half, that is, the sampling frequency of fc, and the sampling frequency fc is recorded in the stream header 821.
FIG. 5 is a structural diagram of a conventional music data receiving apparatus.
As shown in FIG. 5, the music data receiving apparatus 920 comprises a compressed music information analyzing unit 922, a bit stream analyzing unit 923, a basic signal decoding unit 924, a high frequency signal reconstructing unit 925, a band spreading unit 926, a D/A converter 927 and an output control unit 928.
The compressed music information analyzing unit 922 analyzes compressed music data inputted via the transmission path 93.
The bit stream analyzing unit 923 analyzes stream header information.
The basic signal decoding unit 924 decodes the basic part of the bit stream.
The high frequency signal reconstructing unit 925 reconstructs the high frequency signal based on the high frequency information parameter.
The band spreading unit 926 synthesizes the basic signal with the high frequency signal and converts it into time area signal.
The D/A converter 927 converts the digital signal into an analog signal.
The output control unit 928 sets a parameter such as sample frequency or a digital filter coefficient or the like at the D/A converter 927 and the like.
Next, the compressed music data playback processing performed in the music data receiving apparatus 920 will be explained.
FIG. 6 is a flow chart showing the operation of the compressed music data playback processing performed in the music data receiving apparatus 920.
The compressed music information analyzing unit 922 of the music data receiving apparatus 920 waits for receiving the compressed music data sent in a form of the IEC 61937 data format (S91). On receiving the compressed music data, the compressed music information analyzing unit 922 analyzes the information (Pc) of the burst preamble 81 and judges the type of the compressed music data (S92). Here, it is judged as the MPEG 2AAC Low sampling frequency, in other words, the case where the value of the bit 0-4 of Pc is 7 and the value of the bit 8-12 is 1 will be explained. Next, the bit stream analyzing unit 923 analyzes the information of the stream header 821 (S93). The bit stream analyzing unit 923 notifies the output control unit 928 of the sampling frequency recorded in the stream header 821. Output control unit 928 sets a master lock corresponding to the detected sampling frequency and a digital filter coefficient that are detected in the D/A converter 927 and the digital filter coefficient (S94). Next, the basic signal decoding unit 924 starts reading the basic compression stream 822 (S95). After that, the basic signal decoding unit 924 judges whether there is a high frequency parameter or not (S96). This judgment can be made after the basic signal decoding unit 924 finishes reading the last part of the basic compression stream 822 and confirms the presence of the high frequency parameter 823. More specifically, the judgment is made after identifying that a predetermined identifier showing that a high frequency information parameter is stored around the leading part of fill element is set and that it is surely the predetermined identifier by the CRC check.
In the case where there is a high frequency information parameter 823, actual playback is started from the basic signal decoding unit 924 again after setting a sampling frequency that is twice the sampling frequency recorded in the stream header 821 of the output control unit 928.
The AAC-SBR method has a data structure of the bit stream that is the same as a conventional MPEG2 AAC and has a compatibility with a conventional MPEG2 AAC, but no flag indicating that a high frequency information parameter 823 is included in the stream header 821. Also, the high frequency information parameter is recorded in the last part of the basic compression stream.
Therefore, the music data receiving apparatus 920 recognizes the presence of the high frequency compression stream 823 after analyzing the description of the music compressed data up to the last part.
In this way, a parameter for outputting sampling frequency is set in the D/A converter 927 or the like after reading the basic compression stream 822, the high frequency information parameter 823, that is, the burst stream 80 up to the last part, which causes a problem that it takes a lot of time for setting.
Therefore, the present invention aims to provide a receiving apparatus, a sending apparatus and a transmission system of compressed music data that enable instantly judging the presence or absence of high frequency information corresponding to music data to be transmitted in compressed music data transmission.